A document published by the Internal Revenue Service (IRS) that details the tax procedures for aliens - individuals that are not citizens of the United States. Not all aliens are subject to U.S. taxes. Resident aliens, those who have been in the country for a defined period of time, are subject to taxation on their worldwide income just as citizens are. Non-resident aliens are only taxed on income earned within the United States, as well as on certain types of international income.

The most important aspect of IRS Publication 519 is its definition of a taxpayer's status as either a non-resident alien or a resident alien using the Substantial Presence Test, as the applicable tax rules are based on that status. Tax payers may also be considered as dual-resident aliens, and should also determine the tax status of any spouse.